In programming, it’s a common practice to group instructions or statements in smaller and more manageable code blocks. This practice is usually referred as code block organisation. It’s basically breaking down a program or a large section of a program into smaller and logically connected blocks. These blocks are designed to perform a specific task or just to group related functionalities. This approach not only improves code readability but also makes the code more organised and maintainable. Various programming constructs such as functions, methods, try-catch blocks, loops, and conditional statements are commonly used for this purpose.
In SQL, one of the ways to achieve the same is by using Common Table Expression (CTE). In this article, we’ll explore how CTEs can significantly simplify and optimise complex SQL queries.
What is CTE?
CTE, Common Table Expression, is a query that temporarily stores the result set so that it can be referenced and used in another query. The CTE remains available as long as it is within the same execution scope.
In simple terms, a CTE acts like a temporary table that holds the intermediate results of a query, allowing you to use those results later in another SQL query. It is also referred as Subquery Refactoring.
Here, there are two key points to note,
- ‘temporary result set’, which means the output of the CTE is stored temporarily and does not create a permanent table in the database.
- ‘same execution scope’, this refers to the fact that it can only be utilised within the same SQL statements where it is defined. Once that SQL statement is completed, the CTE is no longer accessible, making it confined to its defined scope.
The motive of CTE is to simplify long and complex queries. By breaking down the long queries into simple, smaller and manageable blocks of code, it reduces the complexity while increasing the readability and in some databases, reusability as well.
It is defined by using a WITH clause. The common syntax of a CTE is,